Oxygen inleakage is one of the significant causes of corrosion in power plants, in general, and turbines and steam generators in particular. The concentration of oxygen in the secondary steam/condensate cycle can be changed by both non-corroding and corroding chemical reactions. Hydrazine is added to condensate and it scavenges oxygen by chemical reaction to form water. "Corrosion" hydrogen can also react with oxygen to form water. Organics present in the cycles can scavenge oxygen by chemical reactions thus leading to the formation of carbon dioxide, carbon monoxide and water. Ammonia also reacts with oxygen at high temperatures.
The presence of such volatile oxygen scavengers or reducing agents can alter the metal/metal oxide composition present in the steam/condensate systems and on the surface of materials exposed to the steam cycle. Whether metallic impurities exist in the steam as oxides or as metals is influenced by the oxidation/reduction potential characteristics of the steam. Reducing atmospheres favor the existence of the metallic form and oxidizing atmospheres favor the existence of the metallic oxide forms. Since copper/copper oxides can react with iron/iron oxides and alter their chemical form, the control of the chemical state of copper impurities in the steam is expected to exert a strong influence on the level of iron transport.
To fully optimize the chemistry of both the liquid and gaseous parts of the secondary cycle, for maximum iron/iron oxide stability against chemical reactions and minimum iron transport, it is necessary that chemicals be added to both the condensate and the steam. The introduction of chemical additives to the steam and condensate make it possible to independently modify the chemistry of both the liquid phase and the steam phase. Since corrosion mechanisms operative in the liquid and gaseous phases differ, this dual approach to corrosion control has some level of merit. This approach permits the chemical compensation for impurities formed in the steam generator and not present in the liquid phase, for example, organic acids.
Presently, no methods are employed to monitor and control the oxidation/reduction potential characteristic of steam in today's operating power plants. In such power plants, air inleakage in the steam condenser has been one of the major problems causing power plant failures. The corrosion failure from disc cracking in low pressure steam turbines is also related to air inleakage in the turbine system where the steam pressure becomes lower than the atmosphere pressure. Metal oxidation by free oxygen at high temperatures is a severe problem. However, the intensity of the corrosion process taking place in any steam-water system can also be traced to the variation of hydrogen concentrations in the steam. The appearance of hydrogen in the steam can be due to a number of causes. For example, the corrosion of the metal components under the action of the water or steam, a catalytic decomposition of hydrazine when the boiler feedwater is treated with this reagent, thermal decomposition of organic impurities, and thermal disassociation of the water itself. Of these examples, the main cause is the interaction of water or steam with the steel components. The formation of dense and adherent magnetite film protects the steel from further interaction. Several factors can influence the integrity of the protective film. The disturbance of flow rate, an excess thermal stress associated with the sharp change in local temperature, and the presence of oxidizing deposits in the magnetite film are such factors. Once the protected surface film ruptures, the steam comes into contact with the steel and more hydrogen will appear. It has been a long standing goal to develop a technique to continuously monitor the oxygen and hydrogen concentrations in operating steam systems in order to detect air inleakage and to control corrosion intensities.
It is, therefore, an object of this invention to provide a method for monitoring the oxidation/reduction potential characteristics of a steam environment.
It is a further object of this invention to provide a method for operating the solid electrolyte electrochemical cell so as to make possible the detection of low concentration levels of oxygen and hydrogen in a steam environment.